1. Field of the Invention
The present invention relates to a double-beam light source apparatus suitable for position detection of heterodyne type and a position detecting apparatus of heterodyne type using the double-beam light source apparatus, and is suitably applicable specifically to a high-precision aligning apparatus for aligning a wafer or mask in semiconductor production equipment.
2. Related Background Art
Recently, projection exposure apparatus, which are so called steppers, are frequently used as apparatus for replication of fine pattern for example for semiconductor elements onto a wafer of semiconductor at a high resolution. In particular, since a recent demand is an increase in density of LSI produced by*such apparatus, a finer pattern is desired to be replicated onto a wafer. Higher precision positioning (alignment) is necessary-for replication of finer pattern.
For example, Japanese Laid-open Patent Application No. 62-261003 discloses an apparatus for carrying out high-precision position detection using the heterodyne interference method.
This apparatus utilizes a Zeeman laser, which emits a beam including P-polarized light and S-polarized light having respective frequencies slightly different from each other, as a light source for alignment, in which the beam from the Zeeman laser is split by a polarized beam splitter into P-polarized light of frequency f.sub.1 and S-polarized light of frequency f.sub.2 and the thus split beams are guided via a reflection mirror to irradiate a diffraction grating mark (alignment mark) formed on a reticle (mask) in two predetermined directions. A transparent window is provided at a position adjacent to the diffraction grating mark on the reticle, through which a part of the beams impinging on the diffraction grating mark pass to irradiate a diffraction grating mark formed on a wafer in two predetermined directions.
When the two beams different in frequency from each other thus irradiate the diffraction grating marks respectively in the two directions, diffracted light from each diffraction grating mark is made to pass through a polarizer in detection system as to interfere with each other, whereby two optical beat signals are obtained by photoelectric conversion of interference light by respective photoelectric detectors. A relative phase difference between the two signals corresponds to a deviation amount between the two beams crossing each other on diffraction grating marks and the substrate (reticle or wafer). For example, with a reference signal, which is either one of the optical beat signals detected, the reticle is relatively moved to the wafer such that the phase difference becomes zero or a certain value, whereby high-precision position detection is carried out.
It is, however, difficult for the position detecting apparatus disclosed in Japanese Laid-open Patent Application No. 62-261003 to perfectly separate the P-polarized light from the S-polarized light. For example, the beam of frequency f.sub.2 could be mixed in the beam of frequency f.sub.1 originally expected to be pure to irradiate the diffraction grating marks, which results in degrading the SN ratio of optical beat signals obtained. The degradation of SN ratio raises a problem of lowering the detection precision.
Japanese Laid-open Patent Application No. 2-227604 discloses another position detecting apparatus which can perform position detection at excellent SN ratio, using the heterodyne interference method.
This apparatus is so arranged that a beam from laser source is split into two beams by a beam splitter and that one of the two split beams is made to pass through one of two different acousto-optic modulators (AOM) and the other beam through the other AOM, whereby the two beams have respective frequency differences different from each other. The two beams having the frequency differences different from each other irradiate diffraction grating-marks on reticle and on wafer respectively in two directions. Diffracted light components going out of the diffraction grating marks in the same direction are made interfered with each other. Two optical beat signals are obtained by photoelectric conversion of interference light by respective photoelectric detectors. Relative alignment is achieved at high precision between reticle and wafer using the two optical beat signals. As so arranged, the mixture of beams different in frequency can be avoided so as to enable detection with excellent SN ratio.
In the position detecting apparatus employing the heterodyne interference method as disclosed in above Japanese Laid-open Patent Application No. 2-227604 there are, however, used an optical member (including the beam splitter) for separating a beam from laser source to produce two beams mutually different in frequency (heterodyne beams) and two acousto-optic modulators for producing the frequency differences in the two beams leaving the optical member. Such arrangement is complicated and increases the scale of apparatus, which was inconvenient. Also, a first problem was that the adjustment of optical member was difficult and it was therefore too difficult to keep the precision of position detection within a certain permissible error range in the arrangement in which the two beams different in frequency are produced by provision of two acousto-optic modulators.
Also, in the position detecting apparatus employing the heterodyne interference method as disclosed in above Japanese Laid-open Patent Application No. 2-227604, an optical path difference between the two split beams increases in proportion to wavelength if the two beams (heterodyne beams) different in frequency from each other are produced by splitting a beam from laser source. Therefore, it is theoretically inevitable to use monochromatic light (light of single wavelength), such as laser beam, as the light for position detection. If the alignment is carried out with monochromatic light before replication of circuit pattern on reticle onto a wafer with a resist (photosensitive material) deposited thereon, a second problem of lowering the alignment precision will arise from influence of thin film interference by the resist.
Generally, as a wafer goes through multiple processes, its mark for alignment tends to collapse in cross section, resulting in the cross section being asymmetric. Under such circumstances, the alignment method using interference of monochromatic light such as laser beam had a third problem of lowering the detection precision of position of mark for alignment as the cross section of alignment mark becomes more asymmetric.